This invention relates to broadcast networks, in particular to such networks having multiple antennas that are geographically spread over an area with each antenna serving a region of the area.
Conventional broadcast network planning aims to ensure that for the majority of time, network self-interference is sufficiently controlled to allow reliable reception of the user's target transmissions without undue interference from more distant transmitters radiating different signals on the same frequency, or indeed the same content but outside the interference-free temporal limits of the underlying modulation system.
This can be achieved by the use of multiple frequencies, with signals which are inherently non-interfering. FIG. 1a shows one receiver 14 in a broadcast system receiving signals from two or more transmitters 10, 11 over a broadcast channel 12. FIG. 2 shows an example in which the transmitters use multiple frequencies for interference reduction where within a hexagon network of regions three frequencies are used to ensure no adjacent regions create co-channel interference. The three frequencies are shown with differing shading. As can be seen, each region is bordered by other regions operating at different frequencies. The example of FIG. 2 shows perfectly hexagonal regions, but in reality the coverage of each antenna will vary with geography of a landscape and so this should be considered a schematic representation of a real network.
This can be deemed an n=3 network since three frequencies are needed for full coverage of the area. Generally an n=M network has increasing spectral efficiency as M is made smaller, all other parameters being equal.